Oliver R. W. Pergams of the Chicago Zoological Society and his colleagues originally set out to study the historical relationship between two species of mice native to the Chicago area, the white-footed mouse (see image) and the prairie deer mouse. It soon became apparent from the lack of deer mouse museum specimens, however, that the white-footed mouse had been much more successful over time and had forced the prairie deer mouse from its once dominant position. The researchers thus refocused their study on the white-footed mouse. They analyzed mitochondrial DNA from 56 museum specimens dating back as far as 1855 and compared it to samples from 52 live animals captured in local parks and reserves. When the scientists divided the specimens into 50-year intervals and compared the sequences, they discovered dramatic genetic changes.
Only one mouse caught between 1999 and 2000 had the DNA sequence that was most common among the animals collected prior to 1950, according to the report. In addition, today's predominant sequence only first appeared in 1906. The changes coincide with increased human activity in the area, which the authors propose may have spurred evolutionary change. "We think it's likely that the new gene sequence was either unconditionally advantageous or that it was advantageous relative to environmental changes caused by humans," Pergams comments. Regardless of the cause, he says that the findings suggest "that the 'molecular clock' may sometimes, and sporadically, tick blindingly fast."